The present invention relates to a driving torque (or driving force) distribution control system for a four wheel driving vehicle, and more specifically to a driving torque distribution control system which can ensure the stability of the vehicle by distributing an engine braking force between the front and rear axles during deceleration.
Japanese Patent Provisional Publication No. 61-249859 discloses a vehicle having a 4 WD system and an antiskid brake control system. The 4 WD system of this conventional example has a transfer clutch for switching the drive system from the 2 WD state to the 4 WD state or vice versa. When the brakes are applied, the transfer cltuch is disengaged to disconnect the front wheels from the engine and to bring the drive system to the 2 WD state, and the antiskid brake control system controls the brake of each wheel by using the wheel speed of the undriven front wheels in order to maintain reliable and correct performance of the antiskid brake control without receiving undesired influences of change in torque of the engine and change in rotational inertia due to transmission gear shifting.
In the 4 WD state, the front and rear wheel speeds are synchronized, and the inertia of each wheel is great, so that changes of the wheel speeds become small, and it becomes difficult to determine a quasi vehicle speed from the wheel speeds. This is the reason why the conventional 4 WD system is changed from the 4 WD state to the 2 WD state when the vehicle comes into a decelerating condition or the antiskid control system comes into operation.
However, in the 2 WD state, all the force of engine braking is applied only to the rear wheels. Therefore, the rear wheel speed decreases too much below the front wheel speed, and the rear wheel cornering force becomes lower, so that the tendency to spin of the vehicle is increased especially on a road of a low friction coefficient, such as a road covered with snow or ice, or wetted with rain. If the drive system is put in the 2 WD state in which only the front wheels are driven by the engine, the front wheel cornering force becomes so low that the tendency to drift-out is increased. In either case, the behavior of the 4 WD vehicle becomes unstable during deceleration.